According to the preface, in what ways does a Sadguru accept service from devotees through literary works?

A Sadguru accepts service from devotees through books in two primary ways. As explained in Chapter 0, the first method is when the Sadguru creates profound works themselves, such as the Shri Dnyaneshwari or Shri Nath Bhagavata. The second method involves the Sadguru inspiring their devotees to write their biographies. This allows the writer and the wider congregation of devotees to perform service in the form of dedicated study and devotional readings, known as Parayans, thereby accepting service from both the author and the readers.